1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotography to form images, such as a facsimile machine, a printer, and a multi-functional apparatus having several of these functions, and more particularly, to an image forming apparatus having a registration device to feed a transfer material to a transfer device at a given timing.
2. Description of the Background Art
Image forming apparatuses using electrophotography to form images, such as copiers, printers, facsimile machines, and multi-functional apparatuses having several of these functions, have a transportation system or mechanism to transport a transfer material such as a sheet of recording media onto which the image is transferred. In general, the transportation system may be a roller-based transportation system or a belt-based transportation system.
In either the roller-based transportation system or belt-based transportation system, a difference of sheet transport speed may occur between different module units of the image forming apparatus, such as between a registration device and a transfer device, between a transfer device and a fusing device due to tolerance and/or deviation of roller diameter and/or belt thickness, temperature fluctuation, uneven thickness of transfer material, or the like. This difference of sheet transport speed can occur in almost any type of image forming apparatus, from monochrome machines to color machines, tandem-type machines, four cycle machines, and so on.
Conventionally, differences of sheet transport speed among the module units may be reduced by detecting a temperature fluctuation and/or sheet transport speed, and adjusting the sheet transport speed at the fusing device and/or registration device.
The conventional detection and control process may reduce a difference of a sheet transport speed among the module units by changing the sheet transport speed between a registration device and a transfer device, and/or between a transfer device and a fusing device. However, in sheet transportation between the transfer device and the fusing device, problems such as image scratch or toner scattering at a slack side, and color misalignment, jitter, or density fluctuation at a tensioned side, may occur. Further, due to tolerance and/or deviation of roller diameter and temperature fluctuation at the fusing device, the fluctuation in sheet transport speed increases. Further, it may be necessary to cope with different sheet transportation paths for various types of sheets. Finally, controlling the sheet transport speed using detection alone cannot greatly reduce strain on the sheet, stretched taut as it is between a registration device and a transfer device, or between a transfer device and a fusing device.
In a conventional art, speed increasing phenomenon of the transfer material may occur when a grip of the rear edge of transfer material between a photoconductor and a transfer device is released, and a fluctuation or deviation of a transfer position may occur due to the reduction in tension. This problem may be prevented by using a configuration like that disclosed in JP-2004-117686-A.
The image forming apparatus of JP-2004-117686-A includes a transporting condition detector to detect a transporting condition of the rear edge of a transfer material on a transfer material transporting belt and a fusing device disposed downstream in the direction, in which the transfer material is transported (transporting direction) from the transfer material transporting belt, in which the fusing device presses against the transfer material from both front and back sides for transport, and a control unit to control the transport speed of transfer material at the fusing device based a transporting condition of the rear edge of transfer material detected by the transporting condition detector. With such a configuration, as transportation of a transfer material progresses, and a grip force between the photoconductor and the transfer device decreases while a transporting force of the fusing device increases, the transfer material transport speed is reduced by the fusing device.
Further, as disclosed in JP-H06-80273-A, the amount of slack in the transfer material between the photoconductor and the transfer device is detected. Based on a gap between the detected amount of slack and an adequate amount of slack, the fusing line speed necessary to correct the excess amount of slack is computed, by which image scratch and wrinkles on a fused sheet caused by too much slack due to fluctuation in the fusing line speed can be prevented.
The image forming apparatus of JP-H06-80273-A includes a sensor to detect an amount of slack in a transfer sheet, and a control unit to control a rotation speed of a roller or a fusing roller to transport the transfer sheet. A rotation speed control unit is disposed at a shaft end of the transfer sheet transporting roller or fusing roller so that rotation speed control unit rotates with the roller. The rotation speed control unit is divided into two portions in an axial direction, and at least one portion can slide on the shaft. The rotation speed control unit includes a pulley that can change a space of slanted opposing faces of a V-shaped groove, and a V-belt wound around the pulley and driven at a constant speed by a drive source. By changing the spacing between the opposing slanted faces in the V-shaped groove, the rotation speed of roller shaft can be changed. In other words, when the amount of slack in the transfer sheet detected by the sensor exceeds a given acceptable amount of slack, the rotation speed of the transfer sheet transporting roller or the fusing roller is changed by changing a groove width of pulley so that the slack in the transfer sheet can be adjusted closer to the given acceptable amount of slack. The groove width of the pulley is the fixed when the loosening of transfer sheet achieves the adequate amount of slack.
In another configuration disclosed in JP-2006-195016-A, a moving speed of a belt to transport a recording medium can be matched to a recording medium transport speed of a registration roller or fusing roller, and reliably maintained in such matched condition with high precision without requiring enhanced precision-machining of parts.
In the image forming apparatus of JP-2006-195016-A, a value of rotation of driven roller is obtained in advance when at least one of a belt and a registration roller does not contact a recording sheet, and a target rotation driving speed is determined to minimize the difference with the driven roller rotation information value.
In another configuration disclosed in JP-4264315-B, the line speed of transfer belt and the line speed of registration roller are controlled to prevent color misalignment.
In the image forming apparatus of JP-4264315-B, a rotation speed is computed using first to fourth stations of transfer belt drive system. To maintain a moving speed of transfer belt at a desired speed, a feedback control for transfer process is conducted for a transfer-belt drive motor to variably control the transfer-belt drive motor. In such configuration, the transfer-belt drive motor is variably controlled to maintain the moving speed of transfer belt at a desired speed, or the feedback control for transfer process for variable control of the transfer-belt drive motor is not conducted, which can be selected as required.
Another configuration, disclosed in JP-2008-64891-A, is used to reduce uneven transfer or shock jitter to an image on a recording medium when various types of recording media are used.
In the image forming apparatus of JP-2008-64891-A, toner images on photoconductors Y, M, C, K are sequentially transferred on an intermediate transfer belt to form a superimposed toner image, and a pair of registration rollers feed a transfer sheet to a secondary transfer nip, set by the intermediate transfer belt and a secondary transfer roller, at a given timing. Then, after the leading edge of transfer sheet passes through the secondary transfer nip, a recording medium transporting force by the registration rollers is set smaller than a recording medium transporting force by the registration rollers before the transfer sheet passes through the secondary transfer nip.
However, there are problems with the above-described approaches.
In the image forming apparatus of JP-2004-117686-A, a difference of sheet transport speed between the transfer device and the fusing device may not be reduced so much, and thus the problem of strain on the sheet caused by the difference of sheet transport speed may still remain. Furthermore, because the transporting condition control detector and the control unit to control a transport speed are required, the system becomes more complex as a whole. Essentially the same problems of partial but incomplete solution to the problem of strain as well as increased system complexity attend.
In the image forming apparatus of JP-H06-80273-A, a difference of sheet transport speed between the transfer device and the fusing device may be reduced. However, in such apparatus, a detection slack of is conducted and slack may be solved, but a problem of sheet pulling condition (tug of war of sheet pulling) caused by difference of sheet transport speed may still remain. Furthermore, because the detector to detect slack and the control unit to control a transporting speed based on a detected slack are required, a system becomes more complex as a whole.
In the image forming apparatuses of JP-2006-195016-A and JP-4264315-B, a difference of sheet transport speed between the transfer device and the registration device may be reduced to prevent a fluctuation or deviation of transfer position. However, a difference of sheet transport speed among each one of sheets may not be solved, and thereby a problem of sheet pulling condition (tug of war of sheet pulling) caused by difference of sheet transport speed may still remain. Furthermore, the detector to detect rotation of driven roller, the computing unit to compute a relation of rotation of driven roller and a rotation speed for driving based on a detection result of the detector, the control unit to change a rotation driving speed to a target value based on a computation result of the computing unit are required, by which a system becomes more complex as a whole.
In the image forming apparatus of JP-2008-64891-A, the strain on the sheet may be reduced. However, if a transporting force at the registration device is set small after passing the recording sheet through at a transfer nip, a transporting force at the transfer device may need to be set large enough to transport the recording sheet to the fusing device. In this image forming apparatus, sheet transportation failure may occur when thick paper is used as the recording sheet. Further, if a transfer nip pressure is set too great, problems such as spotty blank areas in the image area and skew caused by nip imbalance at the transfer device may occur, and thereby a sheet cannot be transported reliably. As such, in the conventional arts, the sheet transport speed difference between the module units may be decreased by employing detectors, using predetermined values, or the like. However, such configuration may not be so effective to decrease the sheet transport speed difference because such configuration may cause some time lag for feedback operation for line speed correction.